This invention relates to single lens reflex cameras, motion picture cameras and video cameras, and more particularly to an automatic focusing device in such a photographing device.
In a photographing operation, focusing and exposure are essential. Owing to the recent electronical development, the exposure can be substantially automatically determined, and accordingly the determination of exposure is not a cause that makes pictures unsatisfactory in quality.
On the other hand, a method of automatically focusing a photographing lens on an object (hereinafter referred to merely as "automatic focusing") is not sufficiently practical in use. A short focal length 35 mm camera for the beginner employing such a method has been proposed in the art. However, the camera is not ideal in that it is low in accuracy, it cannot use an interchangeable lens, and it cannot use a long focal length lens.
Recently, almost all the types of motion picture cameras have been combined with sound systems. However, it is considerably difficult for a photographer to operate the motion picture camera while controlling both the focusing of the camera and the adjustment of the sound system. Thus, there has been a strong demand for provision of an automatic focusing device excellent in operability.
In photographing objects with a video camera, a number of matters must be taken into account. For instance, the composition, the movement of a person or persons, and the background must be taken into consideration in operating the video camera. In addition to these matters, the arrangement of color in a natural color image is one of the matters which must be adjusted by the photographer through his experience at all times, because the monitor system of the video camera is a monochrome television receiver.
Recently, the exposure system or mechanism has been improved into an automatic one, and accordingly the photographer can operate a camera more readily. However, it is still considerably difficult for the photographer to precisely control the focusing through the small monitoring finder. Thus, also in this respect, the provision of an automatic focusing device is strongly required.
Automatic focusing devices according to a visitronic module system, an infrared ray system and an ultrasonic wave system are commercially available. Since the automatic focusing devices are not provided according to a system in which the focusing position is detected by utilizing the bundle of rays passing through the photographing optical system (hereinafter referred to as "a TTL system", when applicable), the devices are completely different from an automatic focusing device ideal for the above-described various photographing devices, being disadvantageous in the following points:
(1) The conventional automatic focusing devices are not free from parallax.
(2) Accordingly, it is impossible to display on the finder a part on which the photographing lens should be focused.
(3) It is considerably difficult to apply the conventional automatic focusing devices to a photographing device using a photographing lens such as a zoom lens having a focal length variable over a wide range; more specifically to a video camera, a motion picture camera, or a single lens reflex camera using an interchangeable lens.
(4) The conventional automatic focusing devices are low in accuracy.
An automatic focusing device for practicing the above-described method comprises: intensity distribution detecting means for detecting as video signals the intensity distributions of the images of an object to be photographed which are formed by light beams passing through different portions of a photographing optical system in a photographing device; a signal processing circuit in which the video signals thus detected are stored and while the relation between the video signals being subjected to relative movement little by little, the correlation of the video signals is obtained every movement, and in which the focusing condition of the photographing device is detected from the peak position of the correlation output thereof, and when the peak position is not one for focalization, a drive direction signal for moving a lens in the photographing optical system towards a focusing position is delivered; and lens driving means for moving the lens in a direction specified by the drive direction signal as long as the latter is delivered.
When an object like a grid is photographed with a camera with the above-described automatic focusing device, the correlation output has many peaks. When an object very low in contrast is photographed with the camera, the correlation output is continuously high in level. In both cases, sometimes it is impossible to detect the focusing condition from the peak position of the correlation output.
Accordingly, an object of this invention is to prevent the occurrence of the troubles that, in the above-described cases, the photographing lens is reciprocated, i.e. a hunting phenomenon occurs with the photographing lens, and as the focusing position is not detected, the photographing lens becomes unstable in operation.
More specifically, an object of the invention is to provide an automatic focusing device in which, in the above-described cases, the drive direction signal is made ineffective to stop the photographing lens, and a warning display is carried out.
The nature, principle and utility of the invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawing.